1. Field of the Invention
The present invention relates to a thermal flow sensor for detecting the flow rate of fluid by electrically detecting the amount of heat transmitted to the fluid from a heating element arranged therein. Particularly, the invention relates to a novel improvement in the configuration of a circuit for adjusting the temperature of the heating element.
2. Description of the Related Art
In general, in thermal flow sensors, a heating element and a fluid temperature detection element are arranged in fluid, and a bridge circuit is formed by the heating element, the fluid temperature detection element, and a plurality of resistors, where about in a heating current supplied to the heating element is controlled so as to make the bridge circuit always keep equilibrium.
As a result, the temperature of the heating element is always kept at a control temperature that is higher by a predetermined temperature than the temperature of the fluid detected by the fluid temperature detection element.
However, there exist variations in the heating element, the fluid temperature detection element, the registers, the sensor structure, etc., respectively, so it is necessary to adjust the predetermined temperature for each of individual thermal flow sensors.
In the known thermal flow sensors, to adjust the heating element to the control temperature, the resistance value of at least one of the resistors forming the bridge circuit is adjusted.
As a adjusting method for such a resistance value, there has been widely used a method in which a resistance value necessary to obtain the predetermined temperature is calculated, and a fixed resistor having a resistance value close to the resistance value thus calculated is soldered to the bridge circuit, or a method of using a variable resistor whose resistance value can be changed by a driver or the like, or a laser trimming method in which a part of a membrane resistor is burn off by a laser so as to change its resistance value (see, for example, a first patent document: Japanese patent application laid-open No. 2000-314645).
In the known thermal flow sensors, in case where the fixed resistor is soldered for example, it is necessary to perform the calculation of an optimal resistance value, the selection of an appropriate resistor, and soldering work, so there is a problem that the number of steps required becomes large, and much time is required for adjustment.
In addition, in case where the variable resistor is used, there arises another problem that the resistance value of the variable resistor can be changed due to vibration, etc., thus resulting in a lack of accuracy.
Moreover, in case where laser trimming is applied, the apparatus becomes large in scale, and besides, the resistance value can be adjusted only in a direction from a small resistance value to a large resistance value, so an initial temperature might become too high in consideration of the case where the temperature of the heating element is to be changed in a direction from a high temperature side to a low temperature side, and there is a further problem that when an optimal value has once been exceeded, no readjustment can be made.